As an important technology for image processing and video processing, an upscaling process is known. For example, along with the recent prevalence of displays that are compatible with FHD (Full High Definition) resolution, the upscaling process is necessary to display, with the use of an FHD resolution display, video content having SD (Standard Definition) resolution. In addition, displays compatible with QFHD (Quad Full High Definition) resolution that is superior to FHD resolution have been developed. Furthermore, research has been conducted on displays compatible with UHD (Ultra High Definition) resolution that is superior to QFHD. In line with such increases in display resolution, high performance upscaling process is growing in importance.
Examples of general techniques for upscaling process encompass interpolation methods such as nearest neighbor interpolation, bilinear interpolation, and bicubic interpolation. According to the nearest neighbor interpolation, interpolation is carried out by use of a value of a pixel located nearest to a reference position. Therefore, in a case where an enlargement (zooming) ratio is high, several identical pixels are arranged continuously. This results in substandard tone reproduction, and causes edge parts to be jagged. According to the bilinear interpolation, a pixel value of a reference position is determined by linearly interpolating pixel values of 2×2 pixels (4 pixels) in the vicinity of the reference position. Although the bilinear interpolation is superior in precision to the nearest neighbor interpolation, the bilinear interpolation uses pixel values which have been subjected to linear interpolation. This makes an image seem blurred. According to the bicubic interpolation, a pixel value of a reference position is determined by cubically interpolating pixel values of 4×4 pixels (16 pixels) in the vicinity of a reference position. Although the bicubic interpolation is superior in performance to the nearest neighbor interpolation and the bilinear interpolation, there still occurs a problem of jaggy edge parts with the bicubic interpolation.
Patent Literature 1 discloses a method of carrying out high quality upscaling process of an image and a video without requiring complex processing. According to the upscaling process disclosed in Patent Literature 1, oblique high resolution (HR) pixels are first generated by use of low resolution input pixels which are adjacent in an oblique direction. Next, horizontal high resolution (HR) pixels are generated by use of (i) low resolution input pixels which are adjacent in a horizontal direction and (ii) oblique high resolution (HR) pixels which are adjacent in a vertical direction. Lastly, vertical high resolution (HR) pixels are generated by use of (i) low resolution input pixels which are adjacent in the vertical direction and (ii) oblique high resolution (HR) pixels which are adjacent in the horizontal direction. This causes a high resolution (HR) image to be generated. In addition, while each of the high resolution (HR) pixels is generated, a direction close to an edge direction is controlled to have a large weight, whereas a direction far from the edge direction is controlled to have a small weight. This prevents the occurrence of overshoot, and suppresses the occurrence of jaggy artifacts.